Plastic lead frames for semiconductor devices, packages including same, and methods of fabrication

ABSTRACT

A conductive plastic lead frame and method of manufacturing same, suitable for use in IC packaging. In a preferred embodiment the lead frame is constructed of a plastic or polymer based lead frame structure with an intrinsic conductive polymer coating. In a second embodiment the lead frame is a composite plastic or polymeric material intermixed with an intrinsic conductive polymer coating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to integrated circuit lead frames andmethods of production thereof. In particular, this invention relates toplastic lead frames with a conductive coating or material containedtherein used for packaging integrated circuits and methods ofmanufacturing the same.

2. State of the Art

Integrated circuit (IC) chips are enclosed in plastic packages thatprovide protection from hostile environments and enable electricalinterconnection to printed-circuit boards. During a manufacturingprocess, the IC chip is typically attached to a die paddle of aconventional lead frame or suspended from the lead fingers of aleads-over-chip (LOC) lead frame using an adhesive such as epoxy ordouble-sided tape, and subsequently encapsulated with a dense and rigidplastic by a transfer molding process. In essence, the lead frame formsthe backbone of the molded plastic IC package.

Lead frames typically perform many functions such as: (1) a holdingfixture that indexes with tool-transfer mechanisms as the packageproceeds through various assembly operations, (2) a dam that preventsplastic from rushing out between leads during the molding operation, (3)a chip attach substrate, (4) a support matrix for the plastic, and (5) aelectrical and thermal conductor from chip to board.

Traditionally, lead frames are fabricated from a strip of sheet metal bystamping or chemical milling operations. There are many different metalalloy compositions which are commercially available for producing leadframes. For example, Rao R. Tummala and Eugene J. Rymaszewski,"Microelectronics Packaging Handbook," Table 8-4, 1989, provides 16different alloys available from 9 different manufacturers. Lead framematerial selection depends on many factors such as cost, ease offabrication, strength, thermal conductivity, and matched coefficient ofthermal expansion (CTE). A close match of CTE between the silicon dieand the lead frame is required to avoid chip fracture from differentialexpansion rates.

The most widely used metal for lead frame fabrication is Alloy 42 (42%Nickel--58% Iron). Alloy 42 has a CTE near silicon, and good tensilestrength properties. The disadvantage of Alloy 42 is that it has lowthermal conductivity. Since the lead frame is the main conduit by whichheat flows from the chip to the environment and printed circuit board,this can have a profound effect on the package thermal resistance afterprolonged device operation.

A layered composite strip, such as copper-clad stainless steel, wasdeveloped to emulate the mechanical properties of Alloy 42 whileincreasing thermal conductivity. However, copper-clad stainless steel issomewhat more expensive to manufacture than Alloy 42. When manufacturingcopper-clad stainless steel lead frames, the cladding is accomplished byhigh-pressure rolling of copper foil onto a stainless steel strip,followed by annealing the composite to form a solid-solution weld. Whilecopper alloys provide good thermal conductivity and have a CTE near thatof low-stress molding compounds, there is a substantial CTE mismatchwith respect to silicon.

While numerous alloys have been developed to solve problems with thermalconductivity, CTE mismatch, and strength, other important factors suchas ease of fabrication and cost have not improved as readily.

Conventional methods for making lead frames for integrated circuitdevices are described in U.S. Pat. No. 3,440,027. The use of a plasticsupport structure in a method of forming metal lead frames is describedin U.S. Pat. No. 4,089,733 (hereinafter the "'733" patent). The plasticsupport structure of the '733 patent solves the problem of deformed andmisaligned lead fingers resulting from stress during the bonding processby supporting the lead fingers with a plastic structure. However, the'733 patent requires a metal lead frame in addition to the plasticsupport structure with its attendant costs. A method of manufacturingmulti-layer metal lead frames is disclosed in U.S. Pat. No. 5,231,756(hereinafter the "'756" patent). The '756 patent provides an improvementin aligning power and ground planes for use in a multi-layer lead framewhere such planes are necessary. However, the number of steps requiredto manufacture such multi-layer lead frames will not solve the problemof decreasing costs. In short, none of these related art appear todisclose methods of producing low-cost lead frames made from materialsnot structurally based on metal.

Since packaged ICs are produced in high volumes, a small decrease in thecost per packaged IC can result in substantial savings overall.Accordingly, there is a need in the industry for a low cost plastic leadframe with suitable characteristics for IC packaging.

SUMMARY OF THE INVENTION

The present invention comprises plastic lead frames coated withconductive materials or having conductive materials therein suitable foruse in IC packaging, and methods for fabricating same. The invention maybe used in production of ICs.

By using plastic as the structural base for a lead frame, many costsassociated with the manufacture of metal lead frames can be eliminated.For instance, plastic lead frames can be injection molded or stamped andthen coated with an intrinsic conductive polymer. Furthermore, plasticis intrinsically less expensive as a bulk material than metal alloystypically used in lead frame construction.

Once the lead frame is formed, it can be used in either the conventionaldie attach and connect process or in an LOC process. In the conventionalprocess the die is adhesively attached to a die paddle using epoxy ordouble-sided tape, followed by wire bonding where die pads are connectedto lead fingers from the lead frame. In a LOC process the die could beattached to the lead frame fingers by double-sided adhesive tapefollowed by wire bonding. Alternatively, the LOC process might includedirect connection between the die pads and the LOC lead frame fingerswith a conductive epoxy or Z-axis conductive material using methodscommon in the art.

In the preferred embodiment of the invention, a plastic lead framecoated with an intrinsically conductive polymer is provided. The plasticlead frame structure can be formed by injection molding, stamping oretching from a sheet of plastic or polymer material. This plastic leadframe structure is then coated with a conductive polymer by dipping in asolution or lacquer composed of a polyaniline such as the commerciallyavailable product Ormecon™. By controlling the polyaniline coatingprocess, precise layers with known thicknesses can be produced. Theresulting low cost lead frame has a conductive layer surrounding theplastic structure. Moreover, the CTE of silicon, conductive polymer, andmolding compounds are very nearly matched.

Another embodiment of the present invention is a composite plastic leadframe formed of a conventional polymer intermixed with a conductivepolymer. The composite plastic lead frame structure is formed byinjection molding, stamping or etching from a sheet of the compositeplastic/conductive polymer material. The CTE of the lead frame is wellmatched to that of silicon as in the preferred embodiment, howeverincreased quantities of polyaniline required to provide sufficientconductivity may consequently increase cost relative to the preferredembodiment.

Additional advantages of both the above embodiments are transparency,corrosion resistance, and oxidation resistance. Polyaniline istransparent. By using transparent plastic or polymer in the lead framestructure, ultra violet (UV), or other light source, cure of the dieattach material becomes possible. This is particularly advantageous inan automated production environment. Furthermore, both of the above leadframe embodiments are nonmetallic and thus less susceptible to corrosionor oxidation.

The inventive plastic lead frames solve the problem of reducing costwhile maintaining characteristics necessary for use in commercialproduction of IC packages. The overall cost of IC chip packaging isreduced by using plastic lead frames coated with conductive layers. Useof transparent polymers and intrinsically conductive polymersfacilitates UV or other light source cure of die attach materials.Furthermore, the methods used to produce such lead frames are simple andcan be easily incorporated into existing high-speed production lines formanufacturing IC chips. While, the inventive plastic lead framesdescribed herein have been illustrated with respect to conventional wirebonding and LOC interconnect technology, there is theoretically nolimitation to applying the invention to conventional lead frames,emerging tape automated bonding (TAB) technology, etc. as well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the inventive lead frame in a LOCconfiguration with direct attachment to IC die pads;

FIG. 2 is a blow-up cross-sectional view of the inventive lead frame asit would be attached to an input/output pad of an IC die in the LOCconfiguration;

FIG. 3 is a perspective view of the inventive lead frame in a LOCconfiguration where the IC die is adhesively attached to the leadfingers and wire bonded to die bond pads;

FIG. 4 is a cross-sectional view of the preferred embodiment showing theplastic frame structure with a coating of polyaniline of thickness d;

FIG. 5 is a perspective view of the inventive lead frame in aconventional lead frame configuration with wire bond attachment to ICbond pads to the lead fingers of the lead frame;

FIG. 6 is a cross-sectional view of an embodiment of the presentinvention of an LOC type lead frame in an encapsulated package mountedon a substrate; and

FIG. 7 is a cross-sectional view of another embodiment of the presentinvention of a conventional type lead frame in an encapsulated packagemounted on a substrate.

DETAILED DESCRIPTION OF THE INVENTION

An understanding of the detailed description of the invention isfacilitated by reference to the drawings, FIGS. 1 through 5. Each of thefour embodiments of the invention solve the problem of reducing cost ofproducing lead frames for IC chip packaging. Additionally, at least twoof the embodiments improve the following characteristics: CTE matchingof the lead frame, silicon, and adhesive, anti-corrosion,anti-oxidation, and in-line cure of the die attach adhesive.

Drawing FIG. 1 shows the preferred embodiment of the inventive plasticlead frame 10 as envisioned for application in a LOC packagingconfiguration. The lead fingers 12 of the plastic lead frame 10 (notcompletely shown) are positioned over die pads 14. The lead fingers 12are directly connected to the die pads 14 by an adhesive 16 consistingof a conductive epoxy or Z-axis conductive material. The IC chip or die18 is suspended by the adhesive 16 connecting the lead fingers 12 of thelead frame 10.

Drawing FIG. 2 depicts an enlarged cross-sectional view of the preferredembodiment showing a lead finger 12 as attached to a die bond pad 14 onthe IC die 18. The cross-section of the lead finger 12 is also shownwith the inner lead frame structure 20 coated with a conductive coating22. The conductive coating 22 could be a conductive epoxy, Z-axisconductive material, or any other suitably conductive adhesive known inthe art. The die bond pad 14 is connected to a circuit trace 24 leadingto components (not shown) on the IC die 18. The circuit trace 24 willtypically be underneath a passivation layer of oxide 26 on the IC die18.

Drawing FIG. 3 shows a perspective view of the preferred embodiment in aLOC configuration which utilizes conventional wire bonding. In drawingFIG. 3, lead fingers 12 of the plastic lead frame 10 (not completelyshown) are located over an adhesive tape 28 which holds the IC die 18 tothe lead frame 10. Die bond pads 14 are connected to lead fingers 12 bymeans of wire 30. The wire 30 can be aluminum or gold and is attachedusing wire bonding machines (not shown) well established in the art.

Drawing FIG. 4 shows a magnified cross-section of a plastic lead finger12 of the preferred embodiment of the inventive plastic lead frame 10(not shown). The inner plastic lead frame structure 20 is made of aconventional plastic or polymer material. The surrounding conductivecoating 22 is either an intrinsic conductive polymer such as polyanilineor copper. The polyaniline layer is of thickness "d". The minimumthickness "d", necessary for suitable electrical conductivity isgoverned by the following equation:

    d=1/(πƒσμ).sup.1/2

where f is the maximum frequency of the electrical device, μ is thepermeability of the polyaniline layer, and σ is the conductivity ofpolyaniline layer. For example, where f is 1×10⁹ Hz, σ is 1×10⁵(Ohm.m)⁻¹ and μ is 1.26×10⁻⁶ Henry/m, a thickness of 50 μm is needed forthe polyaniline coating.

Drawing FIG. 5 shows a perspective view of the preferred embodiment in aconventional type lead frame configuration which utilizes conventionalwire bonding. In drawing FIG. 5, lead fingers 112 of the plastic leadframe 100 (not completely shown) are located adjacent the sides 116which holds the IC die 118 to the lead frame 100. Die bond pads 114 areconnected to lead fingers 112 by means of wires 130. The wires 130 canbe aluminum or gold and is attached using wire bonding machines (notshown) well established in the art. The IC die 118 is supported by thedie paddle 120 of the lead frame 110 and is adhesively secured theretoby means of a suitable epoxy adhesive or, alternately, by means of adouble-sided adhesively coated tape.

Drawing FIG. 6 shows an IC die 218 encapsulated by material 230connected to a LOC type lead frame having lead fingers 212 connected tothe bond pads on the active surface of the IC die 218 and connected toelectrical circuits (not shown) on a substrate 220, such as a printedcircuit board or the like. The lead fingers 212 may be shaped in anytype suitable configuration for connections to the IC die 218 and theelectrical circuits of substrate 220. The encapsulating material 230 maybe of any well known suitable type and may include suitable fillermaterial therein.

Drawing FIG. 7 shows an IC die 318 encapsulated by material 340connected to a conventional type lead frame having lead fingers 312 anda die paddle supporting the IC die 318. The lead fingers 312 areconnected to the bond pads on the active surface of the IC die 318 bywires 330 and are connected to electrical circuits (not shown) on asubstrate 320. The lead fingers 312 may be shaped in any type suitableconfiguration for connections to the IC die 318 and the electricalcircuits of substrate 320. The encapsulating material 340 may be anywell known suitable type and may include suitable filler materialtherein.

In the preferred embodiment, plastic lead frames can be dipped in anintrinsically conductive polymer, such as polyaniline, to form aconductive layer. The polyaniline dip could be a dispersion ofpolyaniline in a mixture of organic solvents. The coating is finished bydrying the coating with infra-red heating or baking using techniqueswell known in the art. This dip coating process can be repeated asnecessary to attain the desired thickness. Alternatively, thepolyaniline can be applied in a lacquer dispersion again usingtechniques well known in the art.

In a second embodiment, the polyaniline could be dispersed throughoutthe plastic lead frame structure using commercially availablepolyaniline polymer dispersions such as those offered by ZipperlingKessler & Co. The composite lead frame could be formed by injectionmolding or stamping a sheet of the composite polymer containingpolyaniline.

In both the preferred and second embodiment, such plastic lead framesdescribed have less CTE mismatch with respect to the silicon IC die,plastic mold compounds, and die attach materials relative to traditionalmetal alloys. Furthermore, conductive plastic lead frames are moreflexible than metal alloy lead frames to resist bent leads. Theinventive plastic lead frames can also be made transparent to enableultraviolet radiation cure (or other light source cure) of die attachmaterials. This is a distinct advantage over metal alloys which are nottransparent.

In a third embodiment, the plastic lead frame is formed from aninjection molded or stamped plastic or polymer based sheet of materialto form a lead frame structure. The plastic lead frame structure wouldthen be coated with copper, using electroless copper plating techniqueswell known in the industry. Alternatively, the plastic lead framestructure could be coated with copper using chemical vapor deposition orother plating techniques known in the art. After the copper coating hasbeen applied to the plastic lead frame, the copper may have one or morecoatings or layers of coatings of other conductive metal thereon, suchas a layer of nickel, palladium, silver, gold, other precious metals,etc. In this third embodiment, the resulting low cost plastic lead framehas a conductive layer of copper with suitable electrical and thermalcharacteristics for IC packaging. However, the CTE mismatch between thecopper plated plastic lead frame and the silicon IC die should becomparable to that associated with traditional copper-clad lead frames.

Plastic lead frames can be manufactured by injection molding,compression molding or by stamping to form complex and intricate shapes.There is no particular limitation on the lead frame shape complexityother than the tools used to perform the injection molding or stamping.Furthermore, by reducing the number of steps necessary to produce theplastic lead frame, relative to a metal lead frame, a lower cost can beachieved.

Although the present invention has been described with reference toparticular embodiments, the invention is not limited to these describedembodiments. Rather, the invention is limited only by the appendedclaims, which include within their scope all equivalent devices ormethods which operate according to the principles of the invention asdescribed.

What is claimed is:
 1. A method of manufacturing a plastic lead framefor use in packaging IC dice comprising:forming a plastic lead framestructure; and coating said plastic lead frame structure with aintrinsic conductive polymeric material.
 2. The method of manufacturingin claim 1, wherein said forming a plastic lead frame structure is bystamping or etching from a sheet of polymeric material.
 3. The method ofmanufacturing in claim 1, wherein said forming a plastic lead framestructure is by injection molding plastic or polymer material.
 4. Themethod of manufacturing in claim 1, wherein said forming a plastic leadframe structure is by compression molding plastic or polymer material.5. The method of manufacturing in claim 1, wherein said coating is bydipping said plastic lead frame structure into a solvent dispersion. 6.The method of manufacturing in claim 5, wherein said solvent dispersionfurther comprises an intrinsic conductive polymer.
 7. The method ofmanufacturing in claim 5, wherein said intrinsic conductive polymer is apolyaniline.
 8. A method of manufacturing a plastic lead frame for usein packaging IC dice comprising injection molding composite plasticmaterial further comprising intermixed plastic and intrinsic conductivepolymer.
 9. A method of manufacturing an IC package comprising:forming aconductive plastic lead frame; attaching an IC die to said conductiveplastic lead frame; connecting a plurality of die bond pads on said ICdie to a plurality of lead frame fingers from said conductive plasticlead frame; encapsulating said die and lead frames so attached andconnected; and finishing said electronic package for use.
 10. The methodof claim 9, wherein said conductive plastic lead frame further comprisesa plastic or polymer lead frame structure with a coating of intrinsicconductive polymer.
 11. The method of claim 9, wherein said conductiveplastic lead frame further comprises a polymeric lead frame structurewith a coating of copper.
 12. The method of claim 9, wherein said leadframe structure is injection or compression molded.
 13. The method ofclaim 9, wherein said lead frame structure is stamped from a sheet ofsaid plastic or polymer.
 14. The method of claim 9, wherein said leadframe structure is etched from a sheet of said plastic or polymer. 15.The method of claim 9, wherein said conductive plastic lead frameincludes:an intrinsic conductive polymeric lead frame having a coatingof at least one conductive polymeric material thereon for the formationof an intermetallic connection.
 16. The method of claim 15, wherein saidconductive plastic lead frame includes:a polymeric lead frame having aplurality of coatings of conductive metal layers thereon for theformation of an intermetallic connection.
 17. The method of claim 15,wherein said conductive plastic lead frame includes:a polymeric leadframe having composite metal layers thereon for intermetallicconnections.
 18. The method of claim 15, wherein said conductive plasticlead frame includes:an intrinsic conductive polymeric lead frame havinga coating of at least one conductive metal thereon for the formation ofa metallic connection.
 19. The method of claim 15, wherein the polymericlead frame having a coatings of conductive metal thereon includes acoating of copper and a coating of nickel.
 20. The method of claim 15,wherein the polymeric lead frame having a coatings of conductive metalthereon includes a coating of copper, a coating of nickel, and a coatingof palladium.
 21. The method of claim 15, wherein the polymeric leadframe having a coatings of conductive metal thereon includes a coatingof copper and a coating of silver.
 22. The method of claim 15, whereinthe polymeric lead frame having a coatings of conductive metal thereonincludes a coating of copper and a coating of gold.
 23. A method ofmanufacturing a circuit card comprising attaching one or more ICpackages to a circuit card wherein at least one of said one or more ICpackages contains an intrinsic conductive plastic lead frame.
 24. Themethod of claim 23, wherein at least one of said one or more IC packagescontains a conductive plastic lead frame.